In the past adequate sealing has been achieved, particularly in the case of a longitudinally movable elongate member, using a diaphragm seal where the diaphragm extends between the periphery of the elongate member and a surrounding housing. Such a situation is illustrated in FIG. 1 from which it can be seen that the space 1 under the diaphragm 2 will vary in volume considerably as the elongate member (needle valve member 3) to be sealed is longitudinally displaced in a vertical direction. In the case of controlled dispensing .of a liquid from within the space 1 through a nozzle 4, the pressure within the space 1 will fluctuate giving undesirable fluctuations in the rate of discharge of the liquid introduced along the direction of arrow 5.
An alternative form of seal which has not been subject to this fluctuation in the volume of the space 1 is the stuffing box seal, in this case a chevron shaft seal, shown at 2a in FIG. 2 which is another prior art arrangement. However, the fact that the needle valve member 3 slides relative to the chevron seals of the assembly 2a causes wear in the seals with the result that some of the liquid which has been introduced at 5 into the space 1 can escape vertically upwardly and, particularly in the case of liquids which may be shear-sensitive there is a risk of agglomeration of the material at the seal, with undesirable consequences.